1. Field of the Invention
This invention relates to mixers in general and more particularly to a LTCC double balanced transistor mixer that provides wide bandwidth in a small package size with good isolation.
2. Description of Related Art
A mixer circuit converts a radio frequency (RF) signal to an intermediate frequency (IF) signal which is the difference of the RF and a local oscillator (LO) signal. The IF frequency is obtained by multiplying the RF signal with the local oscillator (LO) signal. The difference or IF frequency is a result of the non-linearity of the mixer.
Below 5 GHz, such mixers are produced using ferrite balun transformers. Unfortunately, the ferrite balun transformer is difficult to repeatedly produce. Variations in tightness of the windings, magnet wire twist rate and permeability/permissivity of the ferrite material cause a lowering of the performance. The ferrite transformers are large and require excessive circuit board space. In addition the ferrite transformers are difficult to assemble to a circuit board requiring a lengthy manual assembly process by a skilled operator. This adds undesirable cost to the product and is difficult for large scale manufacturing. The cost of a unit requiring lengthy manual assembly is dependent upon the country of manufacture.
Balun transformers can also be implemented by transmission lines such as microstrip or stripline on a printed circuit board. Transmission line balun transformers have the disadvantage of requiring large amounts of space for the transmission lines to be run along the printed circuit board. More circuit board space is required for transmission line baluns operating at lower frequencies.
Previous attempts at thick film printable mixers starting from the 800 MHz range are shown in U.S. Pat. Nos. 5,534,830, 5,640,132 and 5,640,699. The mixers shown in these patents are difficult to manufacture because the thickness of the dielectric is hard to maintain at a uniform value. This results in large variations in the electrical parameters of the prior art mixers.
While various mixers have been used, they have suffered from not being able to handle wide bandwidths with high isolation, not having uniform electrical parameters, being difficult to assemble, expensive to produce and requiring excessive circuit board space.
A current unmet need exists for a mixer that is compact, has uniform and consistent electrical parameters that can be assembled at low cost and that can handle wide frequency bandwidths with high isolation.